1. Field of the Invention
The present invention relates to an optical device and its manufacturing method. In particular, the invention relates to an optical modulation device used for optical communication and its manufacturing method.
2. Description of the Related Art
To spread public communication networks using optical fibers, it is important to improve the performance of semiconductor lasers and to increase the yields of semiconductor lasers to manufacture them at low costs.
In particular, to improve the performance of semiconductor lasers, it is indispensable to enable high-speed modulation of laser light to cope with an increase in information quantity. For the purpose of high-speed modulation of laser light, an external modulation scheme is employed to reduce wavelength variation in modulation and thereby enable long-range transmission. In the external modulation scheme, usually, laser light is emitted from a semiconductor laser at constant intensity and then modulated by inputting it to an optical modulator passing or interrupting light (on/off control).
The electro-absorption modulator (hereinafter abbreviated as EAM) is used as an optical modulator in the external modulation scheme. Extinction is attained by using a variation in absorption spectrum due to the Franz-Keldysh effect (in an EAM using a single-layer, thick light absorption layer) or the Stark shift effect (in an EAM using a multiple quantum well structure).
In optical modulators, the degree of absorption of laser light varies depending on the voltage applied. Therefore, if a modulation signal voltage is applied to a high-frequency electric circuit that is connected to an optical modulator, laser light that is emitted from the exit end face of the optical modulator is given intensity modulation corresponding to the signal voltage.
FIG. 11 is a perspective view of a conventional optical device.
In FIG. 11, reference numeral 200 denotes an optical device; 202, an optical modulator; 204, a coplanar line as a transmission line; 224, a termination resistor; 228, bumps; 210, an optical waveguide layer; and 212, laser light (indicated by an arrow) that is input to the optical waveguide layer 210.
FIG. 12 is a perspective view of the conventional optical modulator 202.
In FIG. 12, reference numeral 214 denotes p-side electrode pads; 216, n-side electrode pads.
FIG. 13 is a perspective view of the conventional transmission line 204.
In FIG. 13, reference symbol 204a denotes a signal line of the coplanar line 204; 204b, ground lines of the coplanar line 204; 224, a termination resistor; and 226, a substrate member of the transmission line 204.
Reference numeral 228 denotes conductive bumps that are placed on the signal line 204a and the ground lines 204b. 
Next, an assembling procedure of the conventional optical device 200 will be described.
With the line surface of a coplanar line 204 located above, two bumps 228 are formed on each of the signal line 204a and the ground lines 204b. The bumps 228 are formed at positions corresponding to the p-side electrode pads 214 and the n-side electrode pads 216 of the optical modulator 202.
Thereafter, the front surface of the optical modulator 202 is opposed to the coplanar line 204 and then put on the coplanar line 204 in such a manner that the p-side electrode pads 214 and the n-side electrode pads 216 are placed on the respective bumps 228. The optical modulator 202 and the coplanar line 204 are bonded to each other via the bumps 228 by applying pressure at an increased temperature.
In the conventional optical device 200 that is assembled in the above manner, the bumps 228 have both a role as conductors for electrically connecting the coplanar line 204 and the optical modulator 202 and a role as fixing the optical modulator 202 on the coplanar line 204 and supporting the optical modulator 202.
Having smaller electrical resistance and inductance than conventional bonding wires, the bumps 228 provide an advantage that the optical device 200 is given superior frequency characteristics.
However, because of a variation in ambient temperature that occurs due to heating of peripheral devices of the optical device 200 or depending on the temperature condition of a use environment, the bumps 228 may be thermally deformed (expansion or contraction) during use of the optical modulator 202.
In the conventional optical device 200, since only the bumps 228 have the role of fixing and supporting the optical modulator 202, thermal deformation of the bumps 228 results in a positional variation of the optical modulator 202 that is supported by the bumps 228. As a result, the optical modulator 202 deviates from an optical system that is fixed independently of the optical modulator 202 and deterioration occurs in characteristics due to an optical axis deviation or the like.
As described above, in general, bumps, which have smaller electrical resistance and inductance than bonding wires, can provide an optical device having better high-frequency characteristics than an optical device using bonding wires. In particular, bumps enable formation of an optical device having superior frequency characteristics even in a case where a high-frequency modulation signal is handled as in the case of an optical modulator. However, the conventional optical device 200 has a problem that it cannot satisfy both of superior frequency characteristics and high optical axis stability for preventing deteriorations in characteristics due to an optical axis deviation or the like.
One prior art reference is Japanese Unexamined Patent Publication No. Hei. 10-56163. This publication discloses a photodetector that is mounted on a bare-chip IC via bumps and a bare-chip IC that incorporates a photodetector in a monolithic manner in view of the fact that wire bonding cannot provide stable ultrahigh-speed operation. However, in the former case, the photodetector is connected to the bare-chip IC via the bumps and the bare-chip IC is connected to a fixed side via bumps. Similarly, in the latter case, the bare-chip IC is connected to a fixed side via bumps. In either case, the photodetector side is connected, via the bumps, to a system in which an optical system is fixed and hence the above-described problem cannot be solved.
The present invention has been made to solve the above problem in the art, and an object of the invention is to provide an optical device having superior frequency characteristics and high optical axis stability.
An optical device according to the present invention comprises: an optical device pedestal; an optical modulator having a back surface that is joined to the pedestal and a front surface where a signal electrode pad and a ground electrode pad are arranged; a pair of transmission lines that are provided on the pedestal on both sides of the optical modulator and each of which has a signal line and a ground line on a front surface of a first dielectric substrate; conductive bumps provided on surfaces of the signal line and the ground line of each of the transmission lines and surfaces of the signal electrode pad and the ground electrode pad of the optical modulator, respectively; and a connection transmission line that has a signal connection line and a ground connection line provided on a front surface of a second dielectric substrate, that is oriented in such a manner that the signal connection line and the ground connection line are opposed to the bumps, and that connects, by means of the signal connection line and the ground connection line, the bumps on the electrode pads of the optical modulator with the bumps on the transmission lines.
Accordingly, an optical device according to the present invention is advantageous in that the optical modulator can directly be fixed to the pedestal, a positional deviation from an external optical system can be prevented that would otherwise be caused by a positional variation of the optical modulator due to thermal deformation of the bumps. The use of the bumps makes it possible to prevent deteriorations in optical characteristics due to an optical axis deviation or the like while maintaining superior frequency characteristics. Therefore, an optical device having high optical axis stability can be formed while superior frequency characteristics are maintained.
Another object of the invention is to provide a manufacturing method capable of manufacturing, by a simple process, an optical device having superior frequency characteristics and high optical axis stability.
A manufacturing method of an optical device according to the present invention contains the steps of: preparing an optical device pedestal having a joining portion; joining, to the joining portion of the pedestal, a back surface of an optical modulator having a signal electrode pad and a ground electrode pad on a front surface; providing, on both sides of the joining portion of the pedestal, a pair of transmission lines each having a signal line and a ground line on a front surface of a first dielectric substrate; forming conductive bumps on surfaces of the signal electrode pad and the ground electrode pad of the optical modulator and surfaces of the signal line and the ground line of each of the transmission lines; and orienting a connection transmission line having a signal connection line and a ground connection line on a front surface of a second dielectric substrate in such a manner that the signal connection line and the ground connection line are opposed to the bumps, and connecting the bumps on the electrode pads of the optical modulator with the bumps on the transmission lines by means of the signal connection line and the ground connection line.
Accordingly, a manufacturing method of an optical device according to the present invention is advantageous in that the manufacturing method makes it possible to manufacture an optical device having high optical axis stability by a simple process while maintaining superior frequency characteristics, and that an optical device having high optical axis stability and superior frequency characteristics can be provided at a low cost.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.